The further use and development of the in vivo voltammetric probe will be centered on two general experiments. First, the implanted microelectrode is able to establish the concentrations of the oxidized and reduced species of electroactive drugs injected into the central nervous system. This information can be used not only to monitor the fate of the injected drug, but also to study the redox disposition of brain metabolism. The injected drug acts as an indicator of the brain redox level and will equilibrate with endogenous materials to satisfy the redox potential of the brain. Secondly, the electrode technology and associated electronics are being developed to further reduce the size of the voltammetric probe. With an electrode size of about 10 microns, localized high concentrations of catecholamines and other electroactive endogenous materials could be detected. The electrochemical behavior of both endogenous and injected materials will continue to be compared to their chemistry in vitro.